Drawing processing apparatus, texture processing apparatus, and tessellation method

ABSTRACT

A drawing processing apparatus for performing tessellation processing, comprising a pixel shader and a texture unit. An internal division operation part of the pixel shader issues texture load instructions to the texture unit, specifying parametric coordinate values of a patch of a parametric surface, and thereby acquires internal division factors interpolated based on the parametric coordinate values from the texture unit. The internal division operation part issues texture load instructions to the texture unit further, specifying the internal division factors acquired from the texture unit as new interpolation factors, and thereby acquires control points internally divided based on the internal division factors in advance from the texture unit. Based on the internal division factors corresponding to the parametric coordinate values, the internal division operation part performs recursive internal division operations on the control points acquired from the texture unit, thereby determining a point on the parametric surface corresponding to the parametric coordinate values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates a drawing processing apparatus, a textureprocessing apparatus, and a tessellation method for processing drawingdata.

2. Description of the Related Art

When generating three-dimensional models of drawing objects, free-formsurfaces are often used so as to render more complicated shapes. Amongthe techniques for rendering free-form surfaces, NURBS (Non-UniformRational B-Spline) curves or NURBS surfaces are in widespread use sincethey have the advantage that smooth curves or surfaces can be renderedwith a smaller number of control points. Aside from control points,NURBS has a lot of shape-operating parameters such as weights and knots,which allow local modifications in shape. NURBS also has excellentrendering capabilities such that arcs, straight lines, and conicsections including parabolas can be expressed systematically. Inthree-dimensional computer graphics, technologies for rendering drawingmodels created in NURBS data are being sought after.

Objects smoothly rendered in the form of NURBS, Bezier, or otherparametric surfaces are represented by parameters such as controlpoints. This allows a reduction in the amount of data as compared topolygonal models in which three-dimensional objects are expressed assets of polygons such as triangles. In some applications includingnetwork games, three-dimensional model information is transmitted overnetworks. The object representation using free-form surfaces is suitedeven to such network applications because of the smaller amount of data.

When drawing three-dimensional models rendered in free-form surfaces,the free-form surfaces are divided into polygons, and renderingalgorithms are applied thereto for the sake of drawing processing. Todivide a free-form surface into polygons, the values of parameters inthe equation of the NURBS- or otherwise-defined parametric surface arechanged at predetermined pitches, whereby vertexes on the parametricsurface are determined directly. These points are then connected togenerate the polygons. As a result, the parametric surface is subdividedinto a lot of polygons with a predetermined number of divisions. Thisprocessing is called tessellation processing.

In the tessellation processing, determining the coordinate values of thevertexes on a parametric surface requires that computing be repeated onthe control points that define the shape of the parametric surface. Thistessellation processing, when implemented by software, consumes aconsiderable computing time due to the large amount of calculation. Thetessellation processing is thus not suited to three-dimensional computergraphic applications that need to display the result of drawing in realtime. The tessellation processing is also difficult to implement byhardware since high memory capacities are required in order to store thedata necessary for the tessellation processing and intermediatecalculation results.

Attempts have thus been made to reduce the amount of computation and theamount of memory by limiting the degree of freedom of the surface, bysuch means as decreasing the number of divisions in the tessellationprocessing with a reduction in the amount of computation, reducing thenumber of control points, and using simpler curve equations. Inconsequence, however, the objects to be drawn are rendered in coarsepolygons, which has produced the problem that CG images of sufficientquality cannot be generated.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the foregoingproblems. The present invention relates to a drawing processingtechnology capable of performing tessellation processing efficiently interms of the amount of computation. The present invention also relatesto a drawing processing technology that is advantageous in terms of thememory capacity when performing tessellation processing.

To solve the foregoing problems, one embodiment of the present inventionprovides a drawing processing apparatus for making an approximation toan object to be drawn with a parametric curve represented by coordinatepoints associated with a single parameter. The apparatus comprises: afactor data memory part which stores a group of representative values ofan internal division factor at two points in a domain of the parameterin a one-dimensional parametric coordinate system defined for each ofsegments, or component units, of the parametric curve, the internaldivision factor being given by a linear function of the parameter; aninterpolation processing part which interpolates the group ofrepresentative values of the internal division factor stored in thefactor data memory part by using a linear interpolator with a value ofthe parameter specified in a segment to be processed as an interpolationfactor, thereby calculating an interpolated value of the internaldivision factor corresponding to the specified value of the parameter;and a subdivision processing part which performs an internal divisionoperation on a plurality of control points defining shape of the segmentto be processed at a ratio based on the interpolated value of theinternal division factor, thereby calculating a vertex coordinate on theparametric curve corresponding to the specified value of the parameterin the segment to be processed.

Another embodiment of the present invention is also a drawing processingapparatus. This drawing processing apparatus is one for making anapproximation to an object to be drawn with a parametric surfacerepresented by coordinate points associated with a pair of parameters.The apparatus comprises: a factor data memory part which stores a groupof representative values of a first internal division factor at twopoints in a domain of a first parameter in a two-dimensional parametriccoordinate system defined for each of patches, or component units, ofthe parametric surface, the first internal division factor being givenby a linear function of the first parameter; an interpolation processingpart which interpolates the group of representative values of the firstinternal division factor stored in the factor data memory part by usinga linear interpolator with a value of the first parameter specified in apatch to be processed as an interpolation factor, thereby calculating aninterpolated value of the first internal division factor correspondingto the specified value of the first parameter; and a subdivisionprocessing part which performs an internal division operation on aplurality of control points defining shape of the patch to be processedin an axial direction of the first parameter in the two-dimensionalparametric coordinate system at a ratio based on the interpolated valueof the first internal division factor.

Yet another embodiment of the present invention is also a drawingprocessing apparatus. This apparatus comprises: a texture memory partwhich stores data on a texture; a texture processing part whichinterpolates the data on the texture stored in the texture memory partby using a linear interpolator based on a specified value of aparametric coordinate defined for each of units of drawing processing,thereby calculating interpolated data on the texture corresponding tothe specified value of the parametric coordinate; and a drawingprocessing part which processes the units of drawing processing by usingthe interpolated data on the texture. The units of drawing processingare segments or component units of a parametric curve represented by aone-dimensional parametric coordinate, or patches or component units ofa parametric surface represented by two-dimensional parametriccoordinates. The texture memory part stores a group of representativevalues of an internal division factor at two points in a domain of aparameter as the data on the texture, the internal division factordepending on the parametric coordinate(s) defined for each of thesegments or patches. The texture processing part interpolates the groupof representative values of the internal division factor by using alinear interpolator with values of the parametric coordinate(s)successively specified by the drawing processing part as to a segment orpatch to be processed as interpolation factors, thereby calculatinginterpolated values of the internal division factor corresponding to thespecified values of the parametric coordinate(s). The drawing processingpart performs an internal division operation on a plurality of controlpoints defining shape of the segment or patch to be processed at ratiosbased on the interpolated values of the internal division factor,thereby calculating a vertex coordinate or vertex coordinates on theparametric curve or the parametric surface corresponding to thespecified values of the parametric coordinate(s) in the segment or patchto be processed.

Yet another embodiment of the present invention is a texture processingapparatus. This apparatus comprises: a texture memory part which storesdata on a texture; and a texture processing part which interpolates thedata on the texture stored in the texture memory part by using a linearinterpolator based on a specified value of a parametric coordinatedefined for each of units of drawing processing, thereby calculatinginterpolated data on the texture corresponding to the specified value ofthe parametric coordinate. The units of drawing processing are segmentsor component units of a parametric curve represented by aone-dimensional parametric coordinate, or patches or component units ofa parametric surface represented by two-dimensional parametriccoordinates. The texture memory part stores groups of a plurality ofcontrol points defining shape of the respective segments or patches. Thetexture processing part acquires the group of a plurality of controlpoints to be applied to a segment or patch to be processed from thetexture memory part, and performs an internal division operation thereonby using the linear interpolator, thereby calculating a vertexcoordinate or vertex coordinates on the parametric curve or theparametric surface corresponding to the specified value(s) of theparametric coordinate(s) in the segment or patch to be processed.

Yet another embodiment of the present invention is a data structure of atexture. This data structure of a texture has three-dimensionalcoordinate values of control points defining shape of a parametricsurface are stored as texel values. The texture is configured so that,in response to a texture read instruction accompanied with a specifiedvalue of a parameter in a coordinate system of the parametric surface,the coordinate values of adjoining two of the control points can besampled to calculate interpolated values of the coordinate values of thecontrol points corresponding to the specified value.

Yet another embodiment of the present invention is also a data structureof a texture. This data structure of a texture has initial values andfinal values of internal division factors for use in a recursiveinternal division operation on control points defining shape of aparametric surface are stored as adjoining texel values. The texture isconfigured so that, in response to a texture read instructionaccompanied with a specified value of a parameter in a coordinate systemof the parametric surface, the initial values and the final values ofthe internal division factors can be sampled to calculate interpolatedvalues of the internal division factors corresponding to the specifiedvalue.

Yet another embodiment of the present invention is a tessellationmethod. This tessellation method is one for subdividing a parametriccurve represented by coordinate points associated with a singleparameter into lines. The method comprises: storing in advance aninitial value and a final value of an internal division factor at bothends of a domain of a parameter in a one-dimensional parametriccoordinate system defined for each of segments, or component units, ofthe parametric curve into a factor table, the internal division factorhaving a value depending on the parameter, and interpolating the initialvalue and the final value of the internal division factor stored in thefactor table with a value of the parameter specified in a segment to beprocessed as an interpolation factor, thereby calculating aninterpolated value of the internal division factor corresponding to thespecified value of the parameter; and performing an internal divisionoperation on a plurality of control points defining shape of the segmentto be processed at a ratio based on the interpolated value of theinternal division factor, thereby calculating a vertex coordinate on theparametric curve corresponding to the specified value of the parameterin the segment to be processed.

Yet another embodiment of the present invention is also a tessellationmethod. This tessellation method is one for subdividing a parametricsurface represented by coordinate points associated with a pair ofparameters into polygon meshes. The method comprises: storing in advancean initial value and a final value of a first internal division factorat both ends of a domain of a first parameter in a two-dimensionalparametric coordinate system defined for each of patches, or componentunits, of the parametric surface into a factor table, the first internaldivision factor having a value depending on the first parameter, andinterpolating the initial value and the final value of the firstinternal division factor stored in the factor table with a value of thefirst parameter specified in a patch to be processed as an interpolationfactor, thereby calculating an interpolated value of the first internaldivision factor corresponding to the specified value of the firstparameter; and performing an internal division operation on a pluralityof control points defining shape of the patch to be processed in anaxial direction of the first parameter in the two-dimensional parametriccoordinate system at a ratio based on the interpolated value of thefirst internal division factor.

Incidentally, any combinations of the foregoing components, and anyconversions of expressions of the present invention from/into methods,apparatuses, systems, computer programs, data structures, and the likeare also intended to constitute applicable embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a drawing processing apparatus according toan embodiment;

FIGS. 2A and 2B are diagrams for explaining square primitives to beinput to the rasterizer of FIG. 1;

FIG. 3 is a diagram for explaining a patch, or a minimum component unitof a parametric surface;

FIGS. 4A to 4C are diagrams for explaining the steps for determining apoint on a parametric curve;

FIG. 5 is a diagram showing how the point on the parametric curve isdetermined by repeating internal divisions on four control points;

FIG. 6 is a diagram showing the configuration of a pixel shader and thetexture unit of FIG. 1, intended for tessellation processing;

FIG. 7 is a diagram for explaining the data structure of a control pointtexture of FIG. 6;

FIG. 8 is a diagram for explaining the data structure of an internaldivision factor texture of FIG. 6;

FIG. 9 is a flowchart for explaining the steps of tessellationprocessing by the pixel shader and the texture unit of FIG. 6;

FIG. 10 is a diagram for explaining how interpolated values aredetermined from the initial and final values of internal divisionfactors based on texture load instructions;

FIG. 11 is a diagram for explaining how control points are internallydivided based on texture load instructions;

FIGS. 12A to 12J are diagrams schematically showing the steps fordetermining a point on a parametric surface corresponding totwo-dimensional parametric coordinate values, and tangents at thatpoint;

FIGS. 13A to 13C are diagrams for explaining knot patterns with four,five, and six control points;

FIGS. 14A to 14C are diagrams for explaining knot patterns with seven,eight, and nine control points;

FIG. 15 is a chart summarizing the connection relationship betweensegments for the respective types of knot patterns;

FIGS. 16A to 16C are diagrams for explaining the types of knot patternsto be applied to a parametric surface in each direction of thetwo-dimensional parametric coordinate system;

FIG. 17 is a diagram for explaining the process of calculating theB-spline basis functions of a B-spline curve having the order of 4 andthe number of control points of 4, by using recurrence formulas;

FIG. 18 is a chart showing the process of calculation of FIG. 17,modified by using blending factors; and

FIG. 19 is a chart summarizing the internal division factors for therespective types of knot patterns.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a drawing processing apparatus 100according to an embodiment. The drawing processing apparatus 100performs rendering processing for generating drawing data to bedisplayed on a two-dimensional screen based on three-dimensional modelinformation.

In particular, the drawing processing apparatus 100 of the presentembodiment performs tessellation processing for subdividing an object tobe drawn, modeled with a free-form surface, into a set of geometricprimitives. Initially, the configuration of the drawing processingapparatus 100 will be described along with typical operations thereof.Subsequently, description will be given of the operations pertaining tothe tessellation processing, characteristic to the present embodiment.

A rasterizer 10 acquires vertex data on primitives to be drawn from amemory, another processor, a vertex shader, or the like, and convertsthe data into pixel information corresponding to an intended drawingscreen. Drawing primitives are geometric units of drawing when renderinga three-dimensional object into a polygonal model, such as points,lines, triangles, and rectangles. The data on drawing primitives isexpressed in units of vertexes. The rasterizer 10 performs viewtransformation for transforming drawing primitives in three-dimensionalspace into figures on the drawing plane through projection transform.The rasterizer 10 also scans the figures on the drawing plane in thehorizontal direction of the drawing plane while converting them intoquantized pixels row by row.

In the rasterizer 10, drawing primitives are developed into pixels, andpixel information is calculated for each of the pixels. The pixelinformation includes color values expressed in three primary colors RGB,an alpha (u) value for indicating transparency, a Z value for indicatinga depth, and UV coordinate values or parametric coordinates for makingreference to texture attributes.

The rasterizer 10 performs the foregoing rasterization processing inunits of pixel sets of a predetermined size, each including one or morepixels. The rasterizer 10 buffers rasterized pixel sets while supplyingthem to a shader unit 30 set by set. The pixel sets supplied from therasterizer 10 to the shader unit 30 are subjected to pipeline processingin the shader unit 30. Pixel sets are a collection of pixels for therasterizer 10 to process at a time. They are the units for rasterizationprocessing, and the units for drawing processing in the shader unit 30as well.

The shader unit 30 includes a plurality of pixel shaders 32 which makeasynchronous operations. The pixel shaders 32 process respective pixelsets they are in charge of, thereby performing pixel drawing processingin parallel. The pixel shaders 32 each perform shading processing todetermine the color values of the pixels based on the pixel informationwhich is assigned from the rasterizer 10 in units of the pixel sets.When texture mapping is required, the pixel shaders 32 synthesize colorvalues of textures acquired from a texture unit 70 to calculate thefinal color values of the pixels, and write the pixel data to a memory50.

The texture unit 70 maps texture data to pixels to be processed in eachof the pixel shaders 32. The positions of textures mapped to the pixelson a polygon surface are represented by two-dimensional parametriccoordinates, or a UV coordinate system. The texture unit 70 acquires thecoordinate values (u, v) of textures to be mapped to the pixels from thepixel shaders 32, interpolates the color values corresponding to thecoordinate values (u, v), and supplies the interpolated values to therespective pixel shaders 32.

The pixel shaders 32 also apply such processing as fogging andalpha-blending to the drawing data retained in the memory 50, therebydetermining the final color values of the pixels and updating the pixeldata in the memory 50.

The memory 50 has an area that functions as a frame buffer for storingpixel data generated by the pixel shaders 32 in association with screencoordinates. The pixel data stored in the frame buffer may be eitherthat of a final drawing image or that of an intermediate image in theprocess of shading. The pixel data stored in the frame buffer is outputto and displayed on a display unit.

Next, description will be given of operations of the individualcomponents of the drawing processing apparatus 100, pertaining to thetessellation processing.

The rasterizer 10 receives input of drawing primitives each having asquare shape (hereinafter, referred to simply as square primitive).Square primitives have two vertexes, or the ends of a diagonal of asquare, as parameters. The rasterizer 10 generates two-dimensionalparametric coordinate values (u, v) defined for patches, or minimumcomponent units of a parametric surface, within the square primitivespixel by pixel by utilizing the function of the rasterization processingof converting a square primitive into pixel data.

The rasterizer 10 also determines indexes for consulting control pointdata which defines the shape of the patches of the parametric surface,and the types of knot patterns to be described later which are used inthe u and v directions of the patches, respectively. The resultants aresupplied to the pixel shaders 32 of the shader unit 30 as parameterinformation along with the two-dimensional parametric coordinate values(u, v) in the patches.

FIGS. 2A and 2B are diagrams for explaining square primitives to beinput to the rasterizer 10. FIG. 2A shows square primitives 200 to 203of uniform size to be input to the rasterizer 10. The square primitives200 to 203 each have four rows and four columns of pixel sets eachcontaining a 4×4 matrix of pixels. That is, each of the squareprimitives has a drawing area of 16×16 pixels or a total of 256 pixelseach. In this case, the rasterizer 10 subdivides each of the squareprimitives into 16 rows and 16 columns or a total of 256 ways each, andgenerates two-dimensional parametric coordinate values in succession.

As in FIG. 2A, when the rasterizer 10 receives input of the squareprimitives 200 to 203 of the same size with respect to an object to bedrawn, the dividing granularity of the free-form surface of the objectto be drawn is constant. The free-form surface is thus subdivideduniformly. For adaptive tessellation where the granularity forsubdividing the free-form surface of a drawing object is adjusteddepending on the situation, the sizes of the square primitives arechanged when input to the rasterizer 10.

FIG. 2B is a diagram for explaining square primitives to be input to therasterizer 10 for adaptive tessellation. Square primitives 210 and 211,with two rows and two columns of pixel sets each containing a 4×4 matrixof pixels, have a drawing area of 8×8 pixels or a total of 64 pixels.Here, the rasterizer 10 subdivides each of the square primitives into 8rows and 8 columns or a total of 64 ways each, and generatestwo-dimensional parametric coordinate values in succession.

Meanwhile, a square primitive 212, with five rows and five columns ofpixel sets each containing a 4×4 matrix of pixels, has a drawing area of20×20 pixels or a total of 400 pixels. Here, the rasterizer 10subdivides this square primitive into 20 rows and 20 columns, or a totalof 400 ways, and generates two-dimensional parametric coordinate valuesin succession.

A square primitive 213, having four rows and four columns of pixel setseach containing a 4×4 matrix of pixels, is subdivided into a 16 rows and16 columns or a total of 256 ways as described in FIG. 2A.

As seen above, the greater sizes the square primitives have, the higherthe numbers of divisions are and the more finely the same drawing areasare subdivided. For example, the higher the level of detail for drawinga drawing object at is, the greater sizes the square primitives may begiven for the sake of a larger number of divisions. The sizes of thesquare primitives may also be adjusted to change the number of divisionsaccording to the relationship between the normal directions of thepatches and the view direction. Moreover, when the drawing object has acomplicated shape, the sizes of the square primitives can be increasedfor a larger number of divisions. In this way, the sizes of the squareprimitives may be adjusted depending on the situation, thereby allowingadaptive tessellation processing.

Moreover, parameters for specifying a square primitive can also beadjusted so that a range narrower than the original square area isselected for tessellation processing. With a square primitive, theparameters for specifying the vertexes at both ends of a diagonal of thearea can be adjusted to limit the drawing area. The vertex parameters atboth ends of a diagonal of a square area are (0, 0) and (1, 1), whichmay be changed into values of between 0 and 1 to narrow the drawingarea. As shown by the reference numeral 214, when vertexes (0.25, 0.3)and (0.5, 0.9) are specified, the drawing area of the square primitivecan be narrowed to an oblong area that is defined by the diagonalconnecting these vertexes. The vertex parameters at both ends of thediagonals for defining drawing areas can thus be adjusted to limit thedrawing areas to oblong shapes even if square primitives are input tothe rasterizer 10. This makes it possible to create a surface witharbitrary rectangular areas.

Before discussing the concrete configuration and operation of the shaderunit 30 and the texture unit 70 intended for the tessellationprocessing, an overview will first be given of the steps of thetessellation processing.

Free-form surfaces for rendering three-dimensional objects areclassified into several types including Bezier surfaces and B-splinesurfaces. NURBS (Non Uniform Rational B-Spline) surfaces are used widelyas a more general rendering mode of free-form surfaces. These surfacesare parametric surfaces having two parameters u and v. On the surfaces,points (x, y, z) are defined continuously with respect to the twoparameters (u, v). When an object is modeled with a parametric surface,minimum component units of the parametric surface, called patches, areconnected to express the single surface.

The present embodiment will deal with the case where a NURBS surface,having high design flexibility, is used as the parametric surface. Thesame tessellation processing can also be achieved, however, even withother types of free-form surfaces.

FIG. 3 is a diagram for explaining patches, the minimum component unitsof a parametric surface. Patches are defined within the range of 0≦u≦1and 0≦v≦1. A B-spline surface of order four has a 4 x 4 matrix ofcontrol points arranged in the u direction and v direction, or a totalof 16 control points P₀ to P₁₅. When viewed in the u direction or vdirection, this parametric surface can be regarded as a parametric curvewith a parameter u or v. The minimum units of a parametric curve arecalled segments. A B-spline curve of order four has four control points.

In tessellation processing, the two-dimensional parametric coordinatevalues (u, v) are changed at predetermined pitches with respect to eachof the patches of the parametric surface, so that points on theparametric surface corresponding to the respective two-dimensionalparametric coordinate values (u, v) are determined as the vertexcoordinates of the primitives. By this tessellation processing, thefree-form surface of the object to be drawn is subdivided into meshespatch by patch, and converted into a set of smaller primitives.

To determine a point on the parametric surface corresponding totwo-dimensional parametric coordinate values (u, v), the parametricsurface is initially viewed in the u direction to determine the point onthe parametric curve corresponding to the first parametric coordinatevalue u. Next, the parametric surface is viewed in the v direction todetermine the point on the parametric curve corresponding to the secondparametric coordinate value v.

FIGS. 4A to 4C are diagrams for explaining the steps for determining apoint on a parametric curve. Since the same steps apply to both the udirection and the v direction, a parameter t will be assumed fordescription purpose, without distinction between the first parameter uand the second parameter v of the two-dimensional parametric coordinatesystem.

In the segment of the parametric curve, a point corresponding to theparameter t is determined by performing an internal division operationon the four control points P₀ to P₃, which define the shape of thesegment, three times recursively by using internal division factors thatare dependent on the parameter t.

In the first internal division operation, as shown in FIG. 4A, a pointP₀₁ is determined which divides the first and second control points P₀and P₁ internally at a ratio of a₀₀:(1−a₀₀) based on an internaldivision factor a₀₀ corresponding to the parameter t. Similarly, a pointP₁₂ is determined which divides the second and third control points P₁and P₂ internally at a ratio of a₁₀:(1−a₁₀) based on an internaldivision factor a₁₀. A point P₂₃ is determined which divides the thirdand fourth control points P₂ and P₃ internally at a ratio of a₂₀:(1−a₂₀)based on an internal division factor a₂₀. The three internal divisionfactors a₀₀, a₁₀, and a₂₀ used in the first internal division operationwill be referred to as first internal division factors.

Next, in the second internal division operation, as shown in FIG. 4B, apoint P₀₁₂ is determined which further divides the first and secondinternally dividing points P₀₁ and P₁₂ determined in FIG. 4A internallyat a ratio of a₀₁:(1−a₀₁) based on an internal division factor a₀₁corresponding to the parameter t. Similarly, a point P₁₂₃ is determinedwhich further divides the second and third internally dividing pointsP₁₂ and P₂₃ determined in FIG. 4A internally at a ratio of a₁₁:(1−a₁₁)based on an internal division factor a₁₁. The two internal divisionfactors a₀₁ and a₁₁ used in the second interpolating operation will bereferred to as second internal division factors.

Finally, in the third internal division operation, as shown in FIG. 4C,a point P₀₁₂₃ is determined which further divides the first and secondinternally dividing points P₀₁₂ and P₁₂₃ determined in FIG. 4Binternally at a ratio of a₀₂:(1−a₀₂) based on an internal divisionfactor a₀₂ corresponding to the parameter t. The internal divisionfactor a₀₂ used in the third internal division operation will bereferred to as a third internal division factor.

The point P₀₁₂₃ determined thus by dividing the four control pointsinternally three times recursively falls on the parametric curvecorresponding to the parameter t.

FIG. 5 is a diagram showing how the point P₀₁₂₃ on the parametric curveis determined by repeating internal division of the four control pointsP₀ to P₃. P₀₁(t), P₁₂(t), and P₂₃(t) are the points that divide the fourcontrol points P₀ to P₃ internally with the respective first internaldivision factors corresponding to the parameter t. P₀₁₂(t) and P₁₂₃(t)are the points that further divide the foregoing points internally withthe respective second internal division factors corresponding to theparameter t. P₀₁₂₃(t) is the point that divides the foregoing pointsinternally with the third internal division factor corresponding to theparameter t. When the parameter t is changed, P₀₁₂₃(t) traces theparametric curve L. Incidentally, the tangent to the parametric curve Lat the point P₀₁₂₃(t) is given by a difference vector between the twopoints P₀₁₂(t) and P₁₂₃(t).

The tessellation processing is performed by changing the parameter t inpredetermined small pitches while internally dividing the four controlpoints three times recursively by using the internal division factorscorresponding to the parameter t, thereby determining the pointP₀₁₂₃(t). When points P₀₁₂₃(t) corresponding to the parameter t varyingat the predetermined pitches are connected with lines in succession, theparametric curve within the single segment is approximated by the lines.This processing can be performed in both the u direction and the vdirection, so that the parametric surface within the single patch issubdivided into meshes or a tessellation result of a predeterminednumber of divisions.

FIG. 6 is a diagram showing the configuration of the pixel shaders 32and the texture unit 70 for performing the tessellation processing. Forease of explanation, FIG. 6 shows only a single pixel shader 32representatively. The following description will also deal with thesingle pixel shader 32 alone, whereas the same holds for the rest of thepixel shaders 32.

The pixel shader 32 performs the processing of applying a patch of aparametric surface to the square primitive assigned from the rasterizer10, and subdividing the shape of the object to be drawn into meshes bypatch. The pixel shader 32 divides the patch into meshes by determiningthe points on the parametric surface corresponding to two-dimensionalparametric coordinate values (u, v) generated by the rasterizer 10, asthe vertex coordinates of the meshes. For the sake of this calculation,the pixel shader 32 requires control points that define the shape of thepatch, and data on the internal division factors that are used whenperforming an internal division operation on the control pointsrecursively.

As will be detailed later, the internal division factor to be used wheninternally dividing the control points in the u direction is given by alinear expression of the first parameter u in the two-dimensionalparametric coordinate system. The internal division factor to be usedwhen internally dividing the control points in the v direction is givenby a linear expression of the second parameter v in the two-dimensionalparametric coordinate system.

Suppose that both the first parameter u and the second parameter v arerepresented by t without distinction. The value of an internal divisionfactor corresponding to the parameter t is determined by dividing itsvalues at both ends of an interval 0≦t≦1, or an initial value at t=0 anda final value at t=1, internally at a ratio of t:(1−t) based on theparameter t. That is, the value of the internal division factorcorresponding to any t can be determined if the initial value and thefinal value are given.

This characteristic has an affinity for a linear interpolation functionof the texture unit 70. Handling the initial values and final values ofinternal division factors as texture data, the texture unit 70 can makelinear interpolation between the initial values and final values of theinternal division factors with the parameter t as an interpolationfactor. The internal division factors for an arbitrary parameter t canthus be obtained by utilizing the texture interpolation. This isadvantageous in terms of memory capacity and calculation cost ascompared to such methods that a table containing a list of internaldivision factors for various values of the parameter t is prepared andthat the internal division factors are calculated based on their linearexpressions. Then, in the present embodiment, only the initial valuesand final values of the internal division factors are stored as texturedata in an internal division factor texture 54.

The pixel shader 32 also performs an internal division operation on thecontrol points defining the shape of the patch recursively by usingratios based on the internal division factors. This internal divisionoperation also has an affinity for the linear interpolation function ofthe texture unit 70. Handling the coordinate values of the controlpoints as texture data, the texture unit 70 can make linearinterpolation between two control points by using an internal divisionfactor as the interpolation factor. The points internally divided on thebasis of the internal division factors can thus be obtained by utilizingthe texture interpolation. Then, in the present embodiment, the firstinternal division operation out of the recursive internal divisionoperations on the control points is performed by using the linearinterpolation function of the texture unit 70. For that purpose, thecontrol point data is stored as texture data in a control point texture52.

An internal division operation part 34 of the pixel shader 32 issues atexture load instruction to the texture unit 70, specifyingtwo-dimensional parametric coordinate values (u, v) on the patch of theparametric surface as interpolation factors. The internal divisionoperation part 34 thereby acquires from the texture unit 70 internaldivision factors that are interpolated based on the two-dimensionalparametric coordinate values (u, v). When issuing this texture loadinstruction, the internal division operation part 34 also specifies thetypes of knot patterns to be applied to that patch. The types of knotpatterns are required when the texture unit 70 determines which row ofthe internal division factor texture 54 to refer to.

The internal division operation part 34 also issues a texture loadinstruction to the texture unit 70, specifying the internal divisionfactors acquired from the texture unit 70 as new interpolation factors.The internal division operation part 34 thereby acquires from thetexture unit 70 control points that are interpolated based the internaldivision factors. When issuing this texture load instruction, theinternal division operation part 34 also specifies the index of controlpoint data to be applied to that patch. This index is required when thetexture unit 70 determines which row of the control point texture 52 torefer to.

The internal division operation part 34 performs an internal divisionoperation on the control points acquired from the texture unit 70recursively based on the internal division factors corresponding to thetwo-dimensional parametric coordinate values (u, v). As a result, thepoint on the parametric surface corresponding to the two-dimensionalparametric coordinate values (u, v) is determined as vertex coordinatesof a mesh. For the sake of the recursive internal division operation,the internal division operation part 34 has a register for holdingintermediate calculations. It writes intermediate calculations of anoperation to the register once, and then reads the intermediatecalculations from the register for a further operation.

A vertex data output part 36 writes the vertex coordinate values of amesh of the parametric surface determined by the internal divisionoperation part 34 to the memory 50 in association with thetwo-dimensional parametric coordinate values (u, v). That is, theinternal division operation part 34 changes the two-dimensionalparametric coordinate values (u, v) at predetermined pitches while itdetermines the vertex coordinate values of the meshes on the parametricsurface. The vertex data output part 36 writes the vertex coordinatevalues to the memory 50 in association with the two dimensionalparametric coordinate values (u, v). Consequently, data on theparametric surface in the patch, subdivided in meshes, is formed insidethe memory 50 as tessellation data 56.

The texture unit 70 receives two-dimensional parametric coordinatevalues (u, v) from the pixel shader 32 as interpolation factors,interpolates internal division factors read from the internal divisionfactor texture 54 by using the interpolation factors, and supplies theinterpolated values to the pixel shader 32. The texture unit 70 alsoreceives internal division factors from the pixel shader 32 asinterpolation factors, interpolates control points read from the controlpoint texture 52 by using the interpolation factors, and supplies theinterpolated values to the pixel shader 32.

The control point texture 52 and the internal division factor texture 54consulted by the texture unit 70 store control points and internaldivision factors in the form of a texture, respectively. The textureunit 70 performs linear interpolation on the control points and theinternal division factors with specified interpolation factors byutilizing its function of reading texel values from texture data andperforming linear interpolation thereon by using a linear interpolator.

FIG. 7 is a diagram for explaining the data structure of the controlpoint texture 52. The control point texture 52 has the data structuresuch that 16 control points for defining the shape of patches of aparametric surface are stored as a row of texel values. Each rowcontains the three-dimensional coordinate values (x, y, z) of 16 controlpoints P₀ to P₁₅. The 16 control points P₀ to P₁₅ in each singlehorizontal row are used as control points on a single patch surface. Inthe vertical direction, as many pieces of control point data as thenumber of patch surfaces on the object to be drawn are prepared.

Textures typically contain texel values each consisting of three colorvalues RGB. Here, the coordinate values x, y, and z of the controlpoints are stored instead of the R, G, B values in the texels,respectively, so that the texture data structure is utilized to storethe three-dimensional coordinates of the control points.

The texture unit 70 reads the three-dimensional coordinate values ofcontrol points from the control point texture 52 by specifying areference address which is based on a line number S and a control pointnumber i (i=0, 1, . . . , 15) of the control point texture 52.

FIG. 8 is a diagram for explaining the data structure of the internaldivision factor texture 54. The internal division factor texture 54 hasthe data structure such that the initial values and final values of thefirst, second, and third internal division factors for use in recursiveinternal division operations on control points are stored as texelvalues with respect to each of the knot patterns of the patches on aNURBS surface.

As will be detailed later, in the present embodiment, the knot patternsare limited to nine in number, in consideration of smooth connectivitybetween adjoining patches of a NURBS surface. The rows of the internaldivision factor texture 54 correspond to types #1 to #9 of knotpatterns, respectively. Each row contains the internal division factorsof that type of knot pattern in the form of four texels.

The first texel contains, as its texel values, the initial values of thefirst internal division factors a₀₀, a₁₀, and a₂₀ for use in the firstinternal division operation (a₀₀(0), a₁₀(0), a₂₀(0)) instead of colorvalues (R, G, B). The second texel contains the final values of thefirst internal division factors a₀₀, a₁₀, and a₂₀ for use in the firstinternal division operation (a₀₀(1), a₁₀(1), a₂₀(1)).

The third texel contains, as its texel values, a combination of theinitial values of the second internal division factors a₀₁ and a₁₁ foruse in the second internal division operation and the initial value ofthe third internal division factor a₀₂ for use in the third internaldivision operation (a₀₁(0), a₁₁ (0), a₀₂(0)) instead of color values (R,G, B). The fourth texel contains a combination of the final values ofthe second internal division factors a₀₁ and a₁₁ for use in the secondinternal division operation and the final value of the third internaldivision factor a₀₂ for use in the third internal division operation(a₀₁(1), a₁₁(1), a₀₂(1)).

The texture unit 70 reads internal division factors from the internaldivision factor texture 54 by specifying a reference address which isbased on the type of the knot pattern in the vertical direction and thetexel number in the horizontal direction.

Returning to FIG. 6, description will now be given of the operation inwhich an address generation part 74 and a linear interpolation part 72of the texture unit 70 consult the internal division factor texture 54and the control point texture 52 as texture data and perform linearinterpolation on internal division factors and control points.

The address generation part 74 generates a reference address forconsulting the internal division factor texture 54 based on the type ofthe knot pattern and the two-dimensional parametric coordinate values(u, v) specified by the internal division operation part 34 of the pixelshader 32. The address generation part 74 reads the initial values andfinal values of the internal division factors corresponding to thereference address from the internal division factor texture 54, andinputs the same to the linear interpolation part 72.

The linear interpolation part 72 is a linear interpolator having anadder, a subtracter, a multiplier, and a register for retaining aninterpolation factor. The linear interpolator receives a first inputvalue X, a second input value Y, and an interpolation factor A. Thesubtracter subtracts the first input value X from the second input valueY to calculate the difference between the two input values Y−X, andsupplies it to the multiplier. The multiplier multiplies the differenceY−X input from the subtracter by the interpolation factor A to determinea value A(Y−X), and supplies it to the adder. The adder adds the productA(Y−X) input from the multiplier and the first input value X todetermine a value A(Y−X)+X. The adder outputs the result as the finaloutput value of the linear interpolator.

The output value Z of the linear interpolation part 72 can be modifiedas Z=A(Y−X)+X=(1−A)X+AY. The value is thus one determined by dividingthe first input value X and the second input value Y internally at aratio of A:(1−A) based on the interpolation factor A.

The linear interpolation part 72 performs linear interpolation betweenthe initial values and final values of the respective internal divisionfactors read by the address generation part 74, with the parametriccoordinates u and v specified by the internal division operation part 34as an interpolation factor each. The interpolated values of the internaldivision factors are supplied to the internal division operation part34. Specifically, the linear interpolation part 72 outputs valuesa_(ij)(t)=(1−t)·a_(ij)(0)+t·a_(ij)(1), which are determined by thelinear interpolator interpolating the initial values a_(ij)(0) and finalvalues a_(ij)(1) of the internal division factors a_(ij) with theparameter t as the interpolation factor.

The internal division operation part 34 receives the interpolated valuesof the internal division factors from the linear interpolation part 72.The internal division operation part 34 then specifies the interpolatedvalues of the internal division factors received as new interpolationfactors, thereby requesting from the texture unit 70 the control pointsinternally divided at ratios based on the interpolated values of theinternal division factors. While it usually specifies two-dimensionalparametric coordinate values (u, v) as interpolation factors andrequests from the texture unit 70 texel values interpolated based on thetwo-dimensional parametric coordinate values (u, v), the internaldivision operation part 34 here utilizes the same interface andspecifies the internal division factors as interpolation factors,instead of (u, v) values.

The address generation part 74 generates reference addresses forconsulting the texture, based on the reference row of the control pointdata and the interpolation factors specified by the internal divisionoperation part 34. The address generation part 74 reads the values oftwo control points corresponding to the reference addresses from thecontrol point texture 52, and supplies the values to the linearinterpolation part 72.

The linear interpolation part 72 performs linear interpolation betweenthe two control points read by the address generation part 74, with theinternal division factors specified by the internal division operationpart 34 as interpolation factors. The interpolated values of the controlpoints are supplied to the internal division operation part 34.Specifically, with the internal division factor a_(ij)(t) correspondingto t as an interpolation factor, the linear interpolation part 72outputs coordinate values determined by the linear interpolatorinterpolating the coordinate values (x_(A), y_(A), z_(A)) of a firstcontrol point P_(A) and the coordinate values (x_(B), y_(B), z_(B)) of asecond control point P_(B), i.e., (x_(AB), y_(AB),z_(AB))=(1−a_(ij)(t))−(x_(A), y_(A), z_(A))+a_(ij)(t)·(x_(B), y_(B),z_(B)).

FIG. 9 is a flowchart for explaining the steps of tessellationprocessing by the pixel shader 32 and the texture unit 70.

The pixel shader 32 receives parameter information on the patch to beprocessed from the rasterizer 10. More specifically, the pixel shader 32receives data for specifying the reference row of the control point datato be used for this patch (S10), data for specifying the types of theknot patterns to be used in the respective u and v directions of thepatch (S12), and data for specifying two-dimensional parametriccoordinate values (u, v) with which the patch is divided atpredetermined pitches (S14).

The internal division operation part 34 of the pixel shader 32 issuestexture load instructions with the parameter u as the interpolationfactor, thereby loading the interpolated values a_(ij)(u) correspondingto the parameter u (S16). Receiving a texture load instruction from theinternal division operation part 34, the texture unit 70 reads theinitial values a_(ij)(0) and final values a_(ij)(1) of the internaldivision factors from the row of the internal division factor texture 54corresponding to the type of the knot pattern in the u directionspecified at step S12. The texture unit 70 divides the initial valuesa_(ij)(0) and the final values a_(ij)(1) internally at a ratio of u:1−ubased on the parameter u, thereby determining interpolated valuesa_(ij)(u) of the internal division factors corresponding to theparameter u. The interpolated values a_(ij)(u) of the internal divisionfactors are returned to the internal division operation part 34 as theresult of the texture load instruction.

FIG. 10 is a diagram for explaining how interpolated values a_(ij)(u)are determined from the initial values a_(ij)(0) and final valuesa_(ij)(1) of internal division factors in response to a texture loadinstruction. The internal division operation part 34 supplies (u, type)to the texture unit 70 as the parameters of a texture load instructiontld. Here, “type” represents the index to the type of the knot pattern,and corresponds to the reference row of the internal division factortexture 54.

The address generation part 74 generates the first address of the row ofthe internal division factor texture 54 specified by “type” as thereference address. The address generation part 74 then reads the firsttexel and the second texel of the corresponding row from the referenceaddress, and supplies them to the linear interpolation part 72.Consequently, a group of initial values of the first internal divisionfactors (a₀₀(0), a₁₀(0), a₂₀(0)) and a group of final values of thefirst internal division factors (a₀₀(1), a₁₀(1), a₂₀(1)) are input tothe linear interpolation part 72.

The linear interpolation part 72 outputs values (a₀₀(u), a₁₀(u), a₂₀(u))that are determined by dividing the initial values (a₀₀(0), a₁₀(0),a₂₀(0)) and the final values (a₀₀(1), a₁₀(1), a₂₀(1)) of the firstinternal division factors internally with the parameter u as theinterpolation factor, or at a ratio of u:(1−u). The internal divisionoperation part 34 acquires the interpolated values (a₀₀(u), a₁₀(u),a₂₀(u)) of the internal division factors corresponding to the specifiedparameter u as the return values of the texture load instruction tld.

Next, the internal division operation part 34 also supplies (u′, type)as the parameters of the texture load instruction tld in order toacquire interpolated values of the second and third internal divisionfactors corresponding to the parameter u. Here, u′ has two texels ofoffset added to the value of u. This offset is required in order to skipthe first and second texels in the row of the internal division factortexture 54 specified by “type”, and perform linear interpolation betweenthe third and fourth texels.

The address generation part 74 generates the reference address by addingtwo texels of offset to the first address of the row of the internaldivision factor texture 54 specified by “type”. The address generationpart 74 then reads the third and fourth texels of the corresponding rowfrom the reference address, and supplies them to the linearinterpolation part 72. Consequently, a group of initial values of thesecond and third internal division factors (a₀₁(0), a₁₁(0), a₀₂(0)) anda group of final values of the second and third internal divisionfactors (a₀₁(1), a₁₁(1), a₀₂(1)) are input to the linear interpolationpart 72.

The linear interpolation part 72 outputs values (a₀₁(u), a₁₁(u), a₀₂(u))that are determined by dividing the initial values (a₀₁(0), a₁₁(0),a₀₂(0)) and the final values (a₀₁(1), a₁₁(2), a₀₂(1)) of the second andthird internal division factors internally with the parameter u as theinterpolation factor, or at a ratio of u:(1−u). The internal divisionoperation part 34 acquires the interpolated values (a₀₁(u), a₁₁(u),a₀₂(u)) of the internal division factors corresponding to the specifiedparameter u as the return values of the texture load instruction tld.

The processing of step S16 can be written in pseudo-code as follows:

Load six factors interpolated in the u direction:

tld(u, type)=>a ₀₀(u), a ₁₀(u), a ₂₀(u)

u+2=>u′

tld(u′, type)=>a ₀₁(u), a ₁₁(u), a ₀₂(u)

Here, “=>” indicates that the operation instruction on the left-handside results in the right-hand side.

Returning to FIG. 9, the internal division operation part 34 issues atexture load instruction to load control points that are interpolated inthe u direction at ratios based on the interpolated values a₀₀(u),a₁₀(u), and a₂₀(u) of the first internal division factors acquired atstep S16 (S18). In response to the issuance of the texture loadinstruction by the internal division operation part 34, the texture unit70 acquires pairs of adjoining control points in succession out of thefour control points in the specified reference row of the control pointtexture 52 in the u direction, and supplies them to the linearinterpolation part 72. The linear interpolation part 72 divides thepairs of adjoining control points internally at internal division ratiosbased on the interpolated values a₀₀(u), a₁₀(u), and a₂₀(u) of the firstinternal division factors. The control points divided internally arereturned to the internal division operation part 34 as the result of thetexture load instruction.

FIG. 11 is a diagram for explaining how control points are internallydivided in response to a texture load instruction. Each row of thecontrol point texture 52 contains data on 16 control points specifiedfor a patch to be processed. Here, description will be given of theprocessing on four control points P₀ to P₃ in the u direction. The sameprocessing applies to the other fours of the control points P₄ to P₇, P₈to P₁₁, and P₁₂ to P₁₅ in the u direction when the texels to be readfrom the control point texture 52 are offset in units of four.

The first to fourth texels in the reference row s of the control pointtexture 52 contain the coordinate values of the four control points P₀to P₃. The internal division operation part 34 supplies (a₀₀, s) to thetexture unit 70 as the parameters of a texture load instruction tld.

The address generation part 74 generates the first address of thereference row s of the control point texture 52 as the referenceaddress. The address generation part 74 then reads the first texel andthe second texel of that row from the reference address, and suppliesthem to the linear interpolation part 72. As a result, the coordinatevalues of the first two adjoining control points P₀ and P₁ are input tothe linear interpolation part 72.

The linear interpolation part 72 calculates and outputs the coordinatevalues of a point P₀₁ that divides the input two control points P₀ andP₁ internally at a ratio of a₀₀:(1−a₀₀) based on the first internaldivision factor a₀₀. The internal division operation part 34 acquiresthe coordinate values of the control point P₀₁ internally divided inadvance at the internal division ratio based on the interpolated valuea₀₀(t) of the first internal division factor, as the return values ofthe texture load instruction tld.

Next, the internal division operation part 34 supplies (a₁₀′, s) to thetexture unit 70 as the parameters of a texture load instruction tld.Here, a₁₀′ has a single texel of offset added to the value of a₁₀. Thisoffset is required in order to skip the first texel in the reference rows of the control point texture 54, and make linear interpolation betweenthe second and third texels.

The address generation part 74 generates the reference address by addinga single texel of offset to the first address of the reference row s ofthe control point texture 52. The address generation part 74 then readsthe second and third texels of that row from the reference address, andsupplies them to the linear interpolation part 72. As a result, thecoordinate values of the next two adjoining control points P₁ and P₂ areinput to the linear interpolation part 72.

The linear interpolation part 72 calculates and outputs the coordinatevalues of a point P₁₂ that divides the input two control points P₁ andP₂ internally at a ratio of a₁₀:(1−a₁₀) based on the first internaldivision factor a₁₀. The internal division operation part 34 acquiresthe coordinate values of the control point P₁₂ internally divided inadvance at the internal division ratio based on the interpolated valuea₁₀(t) of the first internal division factor, as the return values ofthe texture load instruction tld.

Moreover, the internal division operation part 34 supplies (a₂₀′, s) tothe texture unit 70 as the parameters of a texture load instruction tld.Here, a₂₀′ has two texels of offset added to the value of a₂₀. Thisoffset is required in order to skip the first and second texels in thereference row s of the control point texture 54, and make linearinterpolation between the third and fourth texels.

The address generation part 74 generates the reference address by addingtwo texels of offset to the first address of the reference row s of thecontrol point texture 52. The address generation part 74 then reads thethird and fourth texels of that row from the reference address, andsupplies them to the linear interpolation part 72. As a result, thecoordinate values of the subsequent two adjoining control points P₂ andP₃ are input to the linear interpolation part 72.

The linear interpolation part 72 calculates and outputs the coordinatevalues of a point P₂₃ that divides the input two control points P₂ andP₃ internally at a ratio of a₂₀:(1−a₂₀) based on the first internaldivision factor a₂₀. The internal division operation part 34 acquiresthe coordinate values of the control point P₂₃ internally divided inadvance at the internal division ratio based on the interpolated valuea₂₀(t) of the first internal division factor, as the return values ofthe texture load instruction tld.

The processing of step S18 can be written in pseudo-code as follows:

Load three control points interpolated:

tld(a ₀₀ , s)=>P ₀₁ xyz

a₁₀+1→a₁₀′

tld(a ₁₀ ′, s)=>P ₁₂ xyz

a₂₀+2→a₂₀′

tld(a ₂₀ ′, s)=>P ₂₃ xyz

Return now to FIG. 9. The internal division operation part 34 performsan internal division operation on the control points that are thusinternally divided in advance, at ratios based on the interpolatedvalues a₀₁(u) and a₁₁(u) of the second internal division factors in theu direction. The internal division operation part 34 performs aninternal division operation further on the results at a ratio based onthe interpolated value a₀₂(u) of the third internal division factor inthe u direction (S20). These internal division operations are performedby register-based computing in the internal division operation part 34.The internal division operation part 34 issues a store instruction st towrite the result of the three internal division operations into theregister.

The processing of step S20 can be written in pseudo-code as follows:

Calculate x-coordinate:

P ₀₁ x+a ₀₁(P ₁₂ x−P ₀₁ x)=>P ₀₁₂ x

P ₁₂ x+a ₁₁(P ₂₃ x−P ₁₂ x)=>P ₁₂₃ x

P ₀₁₂ x+a ₀₂(P ₁₂₃ x−P ₀₁₂ x)=>P ₀₁₂₃ x

Calculate y-coordinate:

P ₀₁ y+a ₀₁(P ₁₂ y−P ₀₁ y)=>P ₀₁₂ y

P ₁₂ y+a ₁₁(P ₂₃ y−P ₁₂ y)=>P ₁₂₃ y

P ₀₁₂ y+a ₀₂(P ₁₂₃ y−P ₀₁₂ y)=>P ₀₁₂₃ y

Calculate z-coordinate:

P ₀₁ z+a ₀₁(P ₁₂ z−P ₀₁ z)=>P ₀₁₂ z

P ₁₂ z+a ₁₁(P ₂₃ z−P ₁₂ z)=>P ₁₂₃ z

P ₀₁₂ z+a ₀₂(P ₁₂₃ z−P ₀₁₂ z)=>P ₀₁₂₃ z

Store the coordinate values:

st P₀₁₂₃x

st P₀₁₂₃Y

st P₀₁₂₃z

Next, the internal division operation part 34 issues texture loadinstructions with the parameter v as the interpolation factor, therebyloading interpolated values b_(ij)(v) of the first, second and thirdinternal division factors corresponding to the parameter v (S22). Aswith the u direction, the texture unit 70, receiving a texture loadinstruction from the internal division operation part 34, reads theinitial values b_(ij)(0) and final values b_(ij)(1) of the internaldivision factors from a row of the internal division factor texture 54corresponding to the type of the knot pattern in the v directionspecified at step S12. The texture unit 70 divides the initial valuesb_(ij)(0) and the final values b_(ij)(1) internally at a ratio of v:1−vbased on the parameter v, thereby determining interpolated valuesb_(ij)(v) of the internal division factors corresponding to theparameter v. The interpolated values b_(ij)(v) of the internal divisionfactors are returned to the internal division operation part 34 as theresult of the texture load instruction.

The operation of step S22 can be written in pseudo-code as follows:

Load six interpolated factors in the v direction:

tld(v, type)=>b ₀₀(v), b ₁₀(v), b ₂₀(v)

v+2=>v′

tld(v′, type)=>b ₀₁(v), b ₁₁(v), b ₀₂(v)

The internal division operation part 34 issues a load instruction ld toread the control points internally divided three times recursively inthe u direction from the register. The internal division operation part34 then performs an internal division operation on the control pointsinternally divided in the u direction, three times recursively in the vdirection at ratios based on the interpolated values b_(ij)(v) of thefirst, second, and third internal division factors determined at stepS22 (S24). The internal division operations in the v direction areperformed in the same manner as in the u direction. Note that while thefirst internal division operation in the u direction is performed byusing the linear interpolation function of the texture unit 70, thethree internal division operations in the v direction are all performedin the internal division operation part 34 by holding intermediatecalculations in the register.

In general, patches of a NURBS surface have different knot patterns inthe u direction and in the v direction. The first, second, and thirdinternal division factors a_(ij)(u) for use in the internal divisionoperations in the u direction and the first, second, and third internaldivision factors b_(ij)(v) for use in the internal division operationsin the v direction are thus independent of each other. That is, internaldivision factors corresponding to different types of knot patterns areused in the u direction and the v direction.

At step S24, the point on the parametric surface that corresponds to thetwo-dimensional parametric coordinate values (u, v) is determinedfinally. The internal division operation part 34 issues a storeinstruction st to store the determined point on the parametric surfaceinto the register.

The processing of step S24 can be written in pseudo-code as follows:

Load four control points calculated in the u direction:

ld P₀x ld P₀y ld P₀z

ld P₁x ld P₁y ld P₁z

ld P₂x ld P₂y ld P₂z

ld P₃x ld P₃y ld P₃z

Calculate x-coordinate:

P ₀ x+b ₀₀(P ₁ x−P ₀ x)=>P ₀₁ x

P ₁ x+b ₁₀(P ₂ x−P ₁ x)=>P ₁₂ x

P ₂ x+b ₂₀(P ₃ x−P ₂ x)=>P ₂₃ x

P ₀₁ x+b ₀₁(P ₁₂ x−P ₀₁ x)=>P ₀₁₂ x

P ₁₂ x+b ₁₁(P ₂₃ x−P ₁₂ x)=>P ₁₂₃ x

P ₀₁₂ x+b ₀₂(P ₁₂₃ x−P ₀₁₂ x)=>P ₀₁₂₃ x

Calculate y-coordinate:

P ₀ y+b ₀₀(P ₁ y−P ₀ y)=>P ₀₁ y

P ₁ y+b ₁₀(P ₂ y−P ₁ y)=>P ₁₂ y

P ₂ y+b ₂₀(P ₃ y−P ₂ y)=>P ₂₃ y

P ₀₁ y+b ₀₁(P ₁₂ y−P ₀₁ y)=>P ₀₁₂ y

P ₁₂ y+b ₁₁(P ₂₃ y−P ₁₂ y)=>P ₁₂₃ y

P ₀₁₂ y+b ₀₂(P ₁₂₃ y−P ₀₁₂ y)=>P ₀₁₂₃ y

Calculate z-coordinate:

P ₀ z+b ₀₀(P ₁ z−P ₀ z)=>P ₀₁ z

P ₁ z+b ₁₀(P ₂ z−P ₁ z)=>P ₁₂ z

P ₂ z+b ₂₀(P ₃ z−P ₂ z)=>P ₂₃ z

P ₀₁ z+b ₀₁(P ₁₂ z−P ₀₁ z)=>P ₀₁₂ z

P ₁₂ z+b ₁₁(P ₂₃ z−P ₁₂ z)=>P ₁₂₃ z

P ₀₁₂ z+b ₀₂(P ₁₂₃ z−P ₀₁₂ z)=>P₀₁₂₃z

Store the coordinate values:

st P₀₁₂₃x

st P₀₁₂₃y

st P₀₁₂₃z

The vertex data output part 36 outputs the coordinate values of thepoint determined on the parametric surface as vertex coordinate valuesof a mesh in association with the two-dimensional parametric coordinatevalues (u, v) (S26).

FIGS. 12A to 12 j are diagrams schematically showing the steps forinternally dividing control points in the u and v directions of aparametric surface, thereby determining the point on the parametricsurface corresponding to two-dimensional parametric coordinate values(u, v), as well as the tangents at that point.

FIG. 12A shows 16 control points defined for a patch of the parametricsurface. Here, the vertical direction of the diagram shall represent theu-axis, and the horizontal direction the v-axis. Four control pointsP₀[j], P₁[j], P₂[j], and P₃[j] are shown in the u direction. Thesubscript j in [ ] represents the column number in the v direction,where j=0 to 3.

FIG. 12B shows the result of the first internal division operation inthe u direction, applied to the 16 control points of FIG. 12A. By thefirst internal division operation in the u direction, each of thecolumns of four control points P₀[j], P₁[j], P₂[j], and P₃[j] arrangedin the u direction are internally divided with the first internaldivision factors a₀₀, a₁₀, and a₂₀. As a result, columns each consistingof three internally dividing points P₀₁[j], P₁₂[j], and P₂₃[j] areobtained. This first internal division operation in the u direction isachieved by the linear interpolation part 72 of the texture unit 70,which performs linear interpolation on the control points stored in thecontrol point texture 52.

FIG. 12C shows the result of the second internal division operation inthe u direction, applied to the result of FIG. 12B. By the secondinternal division operation in the u direction, each of the columns ofthree internally dividing points P₀₁[j], P₁₂[j], and P₂₃[j] arranged inthe u direction are internally divided further with the second internaldivision factors a₀₁ and a₁₁. As a result, columns each consisting oftwo internally dividing points P₀₁₂[j] and P₁₂₃[j] are obtained. Thissecond internal division operation in the u direction is performed bythe internal division operation part 34 of the pixel shader 32.

FIG. 12D shows the result of the third internal division operation inthe u direction, which is applied to the result of FIG. 12C. By thethird internal division operation in the u direction, each of thecolumns of two internally dividing points P₀₁₂[j] and P₁₂₃[j] arrangedin the u direction are internally divided with the third internaldivision factor a₀₂. As a result, columns each consisting of a singlecontrol point P₀₁₂₃[j] are obtained. This third internal divisionoperation in the u direction is also performed by the internal divisionoperation part 34.

The three internal division operations in the u direction are completedthrough the processing of FIGS. 12B to 12D, whereby the four internallydividing points P₀₁₂₃[0], P₀₁₂₃[1], P₀₁₂₃[2], and P₀₁₂₃[3] arranged inthe v direction are obtained.

FIG. 12E shows the result of the first internal division operation inthe v direction, which is applied to the result of FIG. 12D. By thefirst internal division operation in the v direction, the fourinternally dividing points P₀₁₂₃[0], P₀₁₂₃[1], P₀₁₂₃[2], and P₀₁₂₃[3]arranged in the v direction are internally divided with the firstinternal division factors b₀₀, b₁₀, and b₂₀. As a result, threeinternally dividing points P₀₁₂₃ _(—) ₀₁, P₀₁₂₃ _(—) ₁₂, and P₀₁₂₃ _(—)₂₃ are obtained. This first internal division operation in the vdirection is performed by the internal division operation part 34.

FIG. 12F shows the result of the second internal division operation inthe v direction, which is applied to the result of FIG. 12E. By thesecond internal division operation in the v direction, the threeinternally dividing points P₀₁₂₃ _(—) ₀₁, P₀₁₂₃ _(—) ₁₂, and P₀₁₂₃ _(—)₂₃ arranged in the v direction are internally divided with the secondinternal division factors b₀₁ and b₁₁. As a result, two internallydividing points P₀₁₂₃ _(—) ₀₁₂ and P₀₁₂₃ _(—) ₁₂₃ are obtained. Thissecond internal division operation in the v direction is also performedby the internal division operation part 34.

FIG. 12G shows the result of the third internal division operation inthe v direction, which is applied to the result of FIG. 12F. By thethird internal division operation in the v direction, the two internallydividing points P₀₁₂₃ _(—) ₀₁₂ and P₀₁₂₃ _(—) ₁₂₃ arranged in the vdirection are internally divided with the third internal division factorb₀₂. As a result, a single internally dividing point P₀₁₂₃ _(—) ₀₁₂₃ isobtained. This third internal division operation in the v direction isalso performed by the internal division operation part 34.

The three internal division operations in the v direction are completedthrough the processing of FIGS. 12E to 12G, whereby the singleinternally dividing point P₀₁₂₃ _(—) ₀₁₂₃ is obtained finally. This isthe point on the parametric surface corresponding to the two-dimensionalparametric coordinate values (u, v).

In order to determine the tangents at the vertex P₀₁₂₃ _(—hd 0123)finally obtained in the u and v directions, two interpolating operationsin the v direction are applied to the result of FIG. 12C which is giventwo interpolating operations in the u direction.

FIG. 12H shows the result of the first internal division operation inthe v direction, which is applied to the result of FIG. 12C. By thisfirst internal division operation in the v direction, the first row offour internally dividing points P₀₁₂[0], P₀₁₂[1], P₀₁₂[2], and P₀₁₂[3]arranged in the v direction are internally divided with the firstinternal division factors b₀₀, b₁₀, and b₂₀. As a result, threeinternally dividing points P₀₁₂ _(—) ₀₁, P₀₁₂ _(—) ₂₂, and P₀₁₂ _(—) ₂₃are obtained. Similarly, the second row of four internally dividingpoints P₁₂₃[0], P₁₂₃[1], P₁₂₃[2], and P₁₂₃[3] arranged in the vdirection are internally divided with the first internal divisionfactors b₀₀, b₁₀, and b₂₀. As a result, three internally dividing pointsP₁₂₃ _(—) ₀₁, P₁₂₃ _(—) ₁₂, and P₁₂₃ _(—) ₂₃ are obtained. This firstinternal division operation in the v direction is performed by theinternal division operation part 34.

FIG. 12I shows the result of the second internal division operation inthe v direction, which is applied to the result of FIG. 12H. By thissecond internal division operation in the v direction, the first row ofthree internally dividing points P₀₁₂ _(—) ₀₁, P₀₁₂ _(—) ₁₂, and P₀₁₂_(—) ₂₃ arranged in the v direction are internally divided with thesecond internal division factors b₀₁ and b₁₁. As a result, twointernally dividing points P₀₁₂ _(—) ₀₁₂ and P₀₁₂ _(—) ₁₂₃ are obtained.Similarly, the second row of three internally dividing points P₁₂₃ _(—)₀₁, P₁₂₃ _(—) ₁₂, and P₁₂₃ _(—) ₂₃ arranged in the v direction areinternally divided with the second internal division factors b₀₁ andb₁₁. As a result, two internally dividing points P₁₂₃ _(—) ₀₁₂ and P₁₂₃_(—) ₁₂₃ are obtained.

FIG. 12J shows the tangents in the u and v directions at the vertexcoordinates P₀₁₂₃ _(—) ₀₁₂₃ of FIG. 12G, determined from the result ofFIG. 12I. Based on the four internally dividing points of FIG. 12I, theinternal division operation part 34 determines a midpoint U between thetwo top points P₀₁₂ _(—) ₀₁₂ and P₁₂₃ _(—) ₀₁₂, a midpoint D between thetwo bottom points P₁₂₃ _(—) ₀₁₂ and P₁₂₃ _(—) ₁₂₃, a midpoint L betweenthe two left points P₀₁₂ _(—) ₀₁₂ and P₁₂₃ _(—) ₀₁₂, and a midpoint Rbetween the two right points P₀₁₂ _(—) ₁₂₃ and P₁₂₃ _(—) ₁₂₃. Thetangent vector in the u direction at the vertex coordinate P₀₁₂₃ _(—)₀₁₂₃ is given by a difference vector between the bottom midpoint D andthe top midpoint U. The tangent vector in the v direction is given by adifference vector between the left midpoint L and the right midpoint R.

Hereinafter, description will be given in detail that the knot patternsof a NURBS surface can be limited to nine types when connectivitybetween adjoining patches is considered, and that the internal divisionfactors for use in the internal division operations on the controlpoints of the NURBS surface can be given by linear expressions of theparameter t.

The NURBS surface is a parametric surface having two parameters u and v.The shape of each patch of the parametric surface is manipulated by 16control points P₀ to P₁₅, as well as knot vectors which are sequences ofknots that indicate changes in the impact of the respective controlpoints when the parameters u and v vary. When the parametric surface isviewed in the u direction or the v direction, it can be regarded as aparametric curve having a parameter u or v. The shape of each segment ofthe parametric curve is manipulated by four control points in the u or vdirection, as well as a knot vector.

A single parametric surface is formed by generating patches andconnecting adjoining patches to each other. The knot vectors to beapplied to the respective u and v directions of each patch aredetermined so as to secure smooth continuity across the junctions of thepatches. In the present embodiment, the knot patterns to be applied tothe patches are limited to nine types in consideration of theconnectivity between adjoining patches. That is, the free-form surfaceof an object to be drawn is modeled by using a NURBS surface determinedby the nine types of knot patterns, not by using arbitrary NURBSsurfaces.

Since parametric surfaces can be regarded as parametric curves whenviewed in either one of the u and v directions, the followingdescription will deal with the knot patterns of parametric curves. Theknot patterns may be specified in the u direction and in the v directionindependently.

With a B-spline curve of order four, i.e., degree three, a singlesegment is defined by four control points. When defined by five controlpoints, the B-spline curve has two segments. When defined by six controlpoints, the B-spline curve has three segments.

Any one of the nine types of knot patterns is used for each singlesegment. The types and sequence of knot patterns to be used aredetermined depending on the number of control points for defining theshape of the curve, in consideration of the connectivity betweenadjoining segments.

When considering the connectivity between adjoining segments, it is thesmooth continuity at the endpoints of the segments that matters.Focusing attention on a single segment, an endpoint of the segment iscalled open if the endpoint coincides with a control point. If not, theendpoint is called closed. In view of the connectivity of the curve, adistinction is also made as to whether or not the endpoints of segmentsadjoining both ends of the segment in question coincide with controlpoints, i.e., whether the endpoints are open or closed. Moreover, if thesegment in question has a closed endpoint, the knot pattern of thatsegment is determined in consideration of whether the adjoining segmentin connection, sharing the endpoint, has an open or closed endpoint.

The connection relationship between segments shall be expressed by acombination of the following: whether the start point and the end pointof the segment in question are open or closed; whether a segmentadjoining the start-point side of the segment in question has an open orclosed start point; and whether a segment adjoining the end-point sideof the segment in question has an open or closed end point. That is, therelationship shall be expressed in the form of “(open or closed thestart point of the start-point side adjoining segment is):(open orclosed the start point of the segment in question is)-(open or closedthe end point of the segment in question is):(open or closed the endpoint of the end-point side adjoining segment is).”

FIGS. 13A to 13C are diagrams for explaining knot patterns with four,five, and six control points.

FIG. 13A shows the knot pattern to be used when defining a NURBS curveby four control points Q₀ to Q₃. This knot pattern will be referred toas type 2. The start and end points of the segment are open, coincidingwith the control points Q₀ and Q₃, respectively. This knot patternmatches with a Bezier curve.

FIG. 13B shows the knot patterns to be used when defining a NURBS curveby five control points Q₀ to Q₄. In this case, the NURBS curve isrendered by connecting first and second segments at a connection pointE₁.

The start point of the first segment is open, coinciding with thecontrol point Q₀. On the other hand, the end point of the first segmentcoincides with the start point of the second segment continuously, butwith none of the control points. The end point is thus closed. Thesecond segment, the end-point side adjoining segment of the firstsegment, has an open end point which coincides with the control pointQ₄. Consequently, the connection relationship of the first segment isexpressed as “open-closed:open.” This will be referred to as type 3.

The start point of the second segment is in connection with the endpoint of the first segment, coinciding with none of the control points,and thus is closed. The end point of the second segment is open,coinciding with the control point Q₄. The first segment, the start-pointside adjoining segment of the second segment, has an open start pointwhich coincides with the control point Q₀. Consequently, the connectionrelationship of the second segment is expressed as “open:closed-open.”This will be referred to as type 7.

FIG. 13C shows the knot patterns to be used when defining a NURBS curveby six control points Q₀ to Q₅. In this case, the NURBS curve isrendered by connecting first, second, and third segments at twoconnection points E₁ and E₂.

The start point of the first segment is open, coinciding with thecontrol point Q₀. The end point of the first segment is in connectionwith the start point of the second segment, coinciding with none of thecontrol points, and thus is closed. The second segment, the end-pointside adjoining segment of the first segment, has a closed end pointwhich coincides with none of the control points. Consequently, theconnection relationship of the first segment is expressed as“open-closed:closed.” This will be referred to as type 4.

The start point of the second segment is in connection with the startpoint of the first segment, coinciding with none of the control points,and thus is closed. The end point of the second segment is in connectionwith the start point of the third segment, coinciding with none of thecontrol points, and thus is closed. The first segment, the start-pointside adjoining segment of the second segment, has an open start pointwhich coincides with the control point Q₀. The third segment, theend-point side adjoining segment of the second segment, has an open endpoint which coincides with the control point Q₅. Consequently, theconnection relationship of the second segment is expressed as“open:closed-closed:open.” This will be referred to as type 5.

The start point of the third segment is in connection with the end pointof the second segment, coinciding with none of the control points, andthus is closed. The end point of the third segment is open, coincidingwith the control point Q₅. The second segment, the start-point sideadjoining segment of the third segment, has a closed start point.Consequently, the connection relationship of the third segment isexpressed as “closed:closed-open.” This will be referred to as type 6.

FIGS. 14A to 14C are diagrams for explaining knot patterns with seven,eight, and nine or more control points.

FIG. 14A shows the knot patterns to be used when defining a NURBS curveby seven control points Q₀ to Q₆. In this case, the NURBS curve isrendered by connecting first to fourth segments at three connectionpoints E₁ to E₃.

The first segment is of type 4 described previously.

The start point of the second segment is in connection with the startpoint of the first segment, coinciding with none of the control points,and thus is closed. The end point of the second segment is in connectionwith the start point of the third segment, coinciding with none of thecontrol points, and thus is closed. The first segment, the start-pointside adjoining segment of the second segment, has an open start pointwhich coincides with the control point Q₀. The third segment, theend-point side adjoining segment of the second segment, has a closed endpoint which coincides with none of the control points. Consequently, theconnection relationship of the second segment is expressed as“open:closed-closed:closed.” This will be referred to as type 8.

The start point of the third segment is in connection with the end pointof the second segment, coinciding with none of the control points, andthus is closed. The end point of the third segment is in connection withthe start point of the fourth segment, coinciding with none of thecontrol points, and thus is closed. The second segment, the start-pointside adjoining segment of the third segment, has a closed start point.The fourth segment, the end-point side adjoining segment of the thirdsegment, has an open end point which coincides with the control pointQ₆. Consequently, the connection relationship of the third segment isexpressed as “closed:closed-closed:open.” This will be referred to astype 9.

The fourth segment is of type 6 described previously.

FIG. 14B shows the knot patterns to be used when defining a NURBS curveby eight control points Q₀ to Q₇. In this case, the NURBS curve isrendered by connecting first to fifth segments at four connection pointsE₁ to E₄.

The first segment is of type 4, and the second segment is of type 8described previously.

The start point of the third segment is in connection with the end pointof the second segment, coinciding with none of the control points, andthus is closed. The end point of the third segment is in connection withthe start point of the fourth segment, coinciding with none of thecontrol points, and thus is closed. The second segment, the start-pointside adjoining segment of the third segment, has a closed start point.The fourth segment, the end-point side adjoining segment of the thirdsegment, has a closed end point which coincides with none of the controlpoints. Consequently, the connection relationship of the third segmentis expressed as “closed:closed-closed:closed.” This will be referred toas type 1.

The fourth segment is of type 9, and the fifth segment is of type 6described previously.

FIG. 14C shows the knot patterns to be used when defining a NURBS curveby nine or more control points, or ten control points Q₀ to Q9 here. Inthis case, the NURBS curve is rendered by connecting first to sevensegments at six connection points E₁ to E₆. Given n control points, theNURBS curve is generally rendered by connecting (n−3) segments.

Here, the first to seventh segments are of types 4, 8, 1, 1, 1, 9, and6, respectively. With nine or more control points, segments of type 1are thus interposed in the middle.

FIG. 15 summarizes the connection relationship of the segments accordingto the knot patterns of types 1 to 9 which have been described in FIGS.13A to 13C and 14A to 14C. As can be seen, the knot patterns areclassified into nine types depending on whether the endpoints of asegment in question are open or closed, and whether the endpoints ofadjoining segments are open or closed.

FIGS. 16A to 16C are diagrams for explaining the types of knot patternsto be applied to a parametric surface in the u and v directions.

FIG. 16A is a schematic diagram showing a continuous parametric surfaceon which four control points are arranged in the u direction and five inthe v direction. The u direction of this parametric surface correspondsto the case with four control points shown in FIG. 13A. The knot patternof type 2 is thus applied in the u direction. The v direction of theparametric surface, on the other hand, corresponds to the case with fivecontrol points shown in FIG. 13B. The knot pattern of type 3 is thusapplied to the first segment in the v direction, and the knot pattern oftype 7 to the second segment.

FIG. 16B is a schematic diagram showing a continuous parametric surfaceon which eight control points are arranged in the u direction and six inthe v direction. The u direction of this parametric surface correspondsto the case with eight control points shown in FIG. 14B. The knotpatterns of types 4, 8, 1, 9, and 6 are thus applied to the respectivesegments in the u direction in order. Meanwhile, the v direction of thisparametric surface corresponds to the case with six control points shownin FIG. 13C. The knot patterns of types 4, 5, and 6 are thus applied tothe respective segments in order.

FIG. 16C is a table for summarizing the sequences of the types of knotpatterns to be applied, depending on the number of control points in theu and v directions. The rasterizer 10 uses this table to determine thesequences of the types of knot patterns to be applied to individualsegments in the u and v directions of a parametric surface, depending onthe numbers of control points in the u and v directions, respectively.The types of knot patterns determined are supplied to the shader unit 30as parameter information on the individual patches.

Now, description will be given that the internal division factors fordividing control points of a NURBS surface internally are given bylinear expressions.

Initially, description will be given of B-spline curves. A B-splinecurve of order M, or degree (M−1), can be written as the followingequation P(t):

P(t)=N _(0,M)(t)Q ₀ +N _(1,M)(t)Q ₁ + . . . +N _(n,4)(t)Q _(n),

where Q₀, Q₁, . . . , Qn are the control points.

Here, N_(0,M), N_(1,M), . . . N_(n,M) are B-spline basis functions,which are also called blending functions. The B-spline basis functionsN_(j,M) can be determined recursively by the following de Boor-Coxrecurrence formula:

N _(j,M)(t)=(t−t _(j))/(t _(j+M−1) −t _(j))·N _(j,M−1)(t)+(t _(j+M)−t)/(t _(j+M) −t _(j+1))·N _(j+1,M−1)(t),

where N_(j,1)(t)=1 (t_(j)≦t<t_(j+1)), and N_(j,1)(t)=0 (otherwise).

As employed in this recurrence formula, t_(j), t_(j+M), and the like areknots of the parameter t. The B-spline basis functions or blendingfunctions N_(j,M) are determined based on the following knot vector T,which expresses the knots in a sequence of numeric values:

T=[t ₀ , t ₁ , . . . , t _(M+N−1)],

where N is the number of control points, and M is the order. The numberof elements in the knot vector is the sum of the number of controlpoints N and the order M.

Knot vectors T are called uniform if their knots pitches are constant,and called nonuniform if the knot pitches are not constant.

Nonuniform rationalized B-spline curves are NURBS curves. Since NURBScurves have knot vectors T capable of changing the knot pitches, thedifferential coefficients of the curves can be corrected by means of theratios of the knot pitches. This facilitates controlling curveconfiguration, and allows local changes in curve configuration. NURBScurves also have the advantage that they can also render various curvesother than B-spline curves, such as Bezier and Hermite curves, in anidentical form. Moreover, NURBS involves central projection usingrationalized, i.e., homogenous coordinates. This makes it possible toassign weights to the control points, thereby allowing rendering of aperfect circle and the like which cannot be rendered by nonrationalizedB-spline curves.

Furthermore, since the foregoing de Boor-Cox recurrence formula fordetermining the values of blending functions can be applied recursively,computer hardware resources such as registers and arithmetic units canbe suitably used for repetitive operations.

The present embodiment deals with B-spline curves of order four, ordegree three, which are defined by four control points. Description willthus be given of the case where M=4 and N=4. For example, a uniformB-spline curve of order four, with four control points, has a knotvector T of T=[−3, −2, −1, 0, 1, 2, 3, 4].

FIG. 17 is a diagram for explaining the process of calculating theB-spline basis functions of a B-spline curve having the order M of 4 andthe number of control points N of 4, by using the de Boor-Cox recurrenceformula.

The parameter t of the segment shall vary within an interval [0, 1], andthe knot vectors T of all the nine types of knot patterns are defined asT=[*, *, *, 0, 1, *, *, *]. Here, * represents a value that varies withtype. Here, in any of the types of the knot patterns, the B-spline basisfunctions of order one are N_(j,1)=[0, 0, 0, 1, 0, 0, 0].

According to the de Boor-Cox recurrence formula, the jth B-spline basisfunction of order M, or N_(j,M), is a blend of the jth B-spline basisfunction of order (M−1), or N_(j,M−1), and the (j+1)th B-spline basisfunction of order (M−1), or N_(j+1,M−1). Starting from the B-splinebasis functions of order one N_(j,1)=[0, 0, 0, 1, 0, 0, 0], all theB-spline basis functions up to the fourth order can be determined bycalculating those of orders two, three, and four in succession. Thediagram shows the process of this calculation.

For example, from the B-spline basis functions of order one, theB-spline basis functions of order two are calculated as follows:

N _(2,2)(t)=0+(t ₄ −t)/(t ₄ −t ₃)·N _(3,1)(t); and

N _(3,2)(t)=(t−t ₃)/(t ₄ −t ₃)·N _(3,1)(t)+0.

Similarly, from the B-spline basis functions of order two, the B-splinebasis functions of order three are calculated as follows:

N _(1,3)(t)=0+(t ₄ −t)/(t ₄ −t ₂)·N_(2,2)(t);

N _(2,3)(t)=(t−t ₂)/(t ₄ −t ₂)·N _(2,2)(t)+(t₅ −t)/(t ₅ −t₃)·N_(3,2)(t); and

N _(3,3)(t)=(t−t ₃)/(t ₅ −t ₃)·N _(3,2)(t)+0.

For ease of explanation, blending factors of the de Boor-Cox recurrenceformula to be used when blending the jth B-spline basis function oforder (M−1), or and the (j+1)th B-spline basis function of order (M−1),or N_(j+1, M−1), shall be defined as follows:

A _(j,M)=(t−t _(j))/(t _(j+M−1) −t _(j)); and

B _(j,M)=(t _(j+M) −t)/(t _(j+M) −t _(j+1)).

Using the blending factors A_(j,M) and B_(j,M), the de Boor-Coxrecurrence formula can be written as:

N _(j,M)(t)=A _(j,M) ·N _(j,M−1)(t)+B _(j,M) ·N _(j+1,M−1)(t).

FIG. 18 shows the calculation process of FIG. 17, modified by using theblending factors A_(j,M) and B_(j,M). According to this calculationprocess, the B-spline basis functions are determined by the followingrecurrence formulas.

Recurrence formulas for determining the B-spline basis functions oforder four:

N _(0,4)(t)=B _(0,4) N _(1,3)(t);

N _(1,4)(t)=A _(1,4) N _(1,3)(t)+B _(1,4) N _(2,3)(t);

N _(2,4)(t)=A _(2,4) N _(2,3)(t)+B _(2,4) N _(3,3)(t); and

N _(3,4)(t)=A _(3,4) N _(3,3)(t).

Recurrence formulas for determining the B-spline basis functions oforder three:

N _(1,3)(t)=B _(1,3) N _(2,2)(t);

N _(2,3)(t)=A _(2,3) N _(2,2)(t)+B _(2,3) N _(3,2)(t); and

N _(3,3)(t)=A _(3,3) N _(3,2)(t).

Recurrence formulas for determining the B-spline basis functions oforder two:

N _(2,2)(t)=B _(2,2) N _(3,1)(t); and

N _(3,2)(t)=A _(3,2) N _(3,1)(t).

The B-spline basis function of order one is given by

N _(3,1)(t)=1.

When the foregoing recurrence formulas for determining the B-splinebasis functions of order four are applied, the equation P(t) of theB-spline curve can be modified as follows:

$\begin{matrix}{{P(t)} = {{Q_{0}{N_{0,4}(t)}} + {Q_{1}{N_{1,4}(t)}} + {Q_{2}N_{2,4}} + {Q_{3}{N_{3,4}(t)}}}} \\{= {{Q_{0}\left\{ {B_{0,4}{N_{1,3}(t)}} \right\}} + {Q_{1}\left\{ {{A_{1,4}{N_{1,3}(t)}} + {B_{1,4}{N_{2,3}(t)}}} \right\}} +}} \\{{{Q_{2}\left\{ {{A_{2,4}{N_{2,3}(t)}} + {B_{2,4}{N_{3,3}(t)}}} \right\}} + {Q_{3}\left\{ {A_{3,4}{N_{3,3}(t)}} \right\}}}} \\{= {{{N_{1,3}(t)}\left\{ {{Q_{0}B_{0,4}} + {Q_{1}A_{1,4}}} \right\}} + {{N_{2,3}(t)}\left\{ {{Q_{1}B_{1,4}} + {Q_{2}A_{2,4}}} \right\}} +}} \\{{{N_{3,3}(t)}{\left\{ {{Q_{2}B_{2,4}} + {Q_{3}A_{3,4}}} \right\}.}}}\end{matrix}$

Since B_(0,4)+A_(1,4)=(t₄−t)/(t₄−t₁)+(t−t₁)/(t₄−t₁)=1, it can be seenthat the calculation Q₀B_(0,4)+Q₁A_(1,4) is equivalent to determiningthe point Q₀₁ that divides the two points Q₀ and Q₁ internally at aratio of A_(1,4):B_(0,4).

In general,B_(j,M)+A_(j+1,M)=(t_(j+M)−t)/(t_(j+M)−t_(j+1))+(t−t_(j+1))/(t_(j+M)−t_(j+1))=1.Thus, the calculation Q₁B_(1,4)+Q₂A_(2,4) is equivalent to determiningthe point Q₁₂ that divides the two points Q₁ and Q₂ internally at aratio of A_(2,4):B_(1,4). The calculation Q₂B_(2,4)+Q₃A_(3,4) isequivalent to determining the point Q₂₃ that divides the two points Q₂and Q₃ internally at a ratio of A_(3,4):B_(2,4).

Hence, using the internally dividing points Q₀₁, Q₁₂, and Q₂₃, theequation P(t) can be written as:

P(t)=N _(1,3)(t)Q ₀₁ +N _(2,3)(t)Q ₁₂ +N _(3,3)(t)Q ₂₃.

When the recurrence formulas for determining the B-spline basisfunctions of order three are applied further, this equation can bemodified as follows:

$\begin{matrix}{{P(t)} = {{Q_{01}\left\{ {B_{1,3}{N_{2,2}(t)}} \right\}} + {Q_{12}\left\{ {{A_{2,3}{N_{2,2}(t)}} + {B_{2,3}{N_{3,2}(t)}}} \right\}} +}} \\{{Q_{23}\left\{ {A_{3,3}{N_{3,2}(t)}} \right\}}} \\{= {{{N_{2,2}(t)}\left\{ {{Q_{01}B_{1,3}} + {Q_{12}A_{2,3}}} \right\}} + {{N_{3,2}(t)}{\left\{ {{Q_{12}B_{2,3}} + {Q_{23}A_{3,3}}} \right\}.}}}}\end{matrix}$

Here, the calculation Q₀₁B_(1,3)+Q₁₂A_(2,3) is equivalent to determiningthe point Q₀₁₂ that divides the two points Q₀₁ and Q₁₂ internally at aratio of A_(2,3):B_(1,3). The calculation Q₁₂B_(2,3)+Q₂₃A_(3,3) isequivalent to determining the point Q₁₂₃ that divides the two points Q₁₂and Q₂₃ internally at a ratio of A_(3,3):B_(2,3).

Using the internally dividing points Q₀₁₂ and Q₁₂₃, the equation P(t)can thus be written as:

P(t)=N _(2,2)(t)Q ₀₁₂ +N _(3,2)(t)Q ₁₂₃.

When the recurrence formulas for determining the B-spline basisfunctions of order two are applied further, this equation can bemodified as follows:

P(t)=Q ₀₁₂ B _(2,2) N _(3,1)(t)+Q ₁₂₃ A _(3,2) N _(3,1)(t)=N _(3,1)(t){Q₀₁₂ B _(2,2) +Q ₁₂₃ A _(3,2)}.

Here, the calculation Q₀₁₂B_(2,2)+Q₁₂₃A_(3,2) is equivalent todetermining the point Q₀₁₂₃ that divides the two points Q₀₁₂ and Q₁₂₃ ata ratio of A_(3,2):B_(2,2).

Using the internally dividing point Q₀₁₂₃, the equation P(t) can thus bewritten as:

P(t)=N _(3,1)(t)Q ₀₁₂₃.

Since the B-spline basis function of order one N_(3,1)(t)=1, thenP(t)=Q₀₁₂₃.

The foregoing shows that the point P(t) on the B-spline curve of orderfour, corresponding to the parameter t, can be determined by repeatinginternal division calculations between adjoining points onto the fourcontrol points Q₀, Q₁, Q₂, and Q₃ three times recursively.

More specifically, in the first internal division calculation, internaldivisions are calculated between adjoining control points out of thefour control points Q₀, Q₁, Q₂, and Q₃, whereby the three internallydividing points Q₀₁, Q₁₂, and Q₂₃ are determined. In the secondcalculation, internal divisions are calculated between adjoininginternally dividing points out of the three internally dividing pointsQ₀₁, Q₁₂, and Q₂₃, whereby two internally dividing points Q₀₁₂ and Q₁₂₃are determined. In the third calculation, an internal division betweenthe two internally dividing points Q₀₁₂ and Q₁₂₃ is calculated todetermine the internally dividing point Q₀₁₂₃. The internally dividingpoint Q₀₁₂₃ resulting from the third internal division calculation isthe point P(t).

The foregoing method of determining a point on a B-spline curve bydividing control points internally in a recursive fashion is anapplication of a calculation method known as de Casteljau's algorithm.The de Casteljau's algorithm is commonly known as a calculation methodfor Bezier curves. It can be seen that the de Casteljau's algorithm isalso applicable to B-spline curves when the de Boor-Cox recurrenceformulas are modified thus.

In summary, the recurrence formulas for determining a point on aB-spline curve of order four from four control points Q₀, Q₁, Q₂, and Q₃are the following:

First internal division calculation:

Q ₀₁=(t ₄ −t)/(t ₄ −t ₁)·Q ₀+(t−t ₁)/(t ₄ −t ₁)·Q₁;

Q ₁₂=(t ₅ −t)/(t ₅ −t ₂)·Q ₁+(t−t ₂)/(t ₅ −t ₂)·Q ₂; and

Q ₂₃=(t ₆ −t)/(t ₆ −t ₃)·Q ₂+(t−t ₃)/(t ₆ −t ₃)·Q ₃.

Second internal division calculation:

Q ₀₁₂=(t ₄ −t)/(t ₄ −t ₂)·Q ₀₁+(t−t ₂)/(t ₄ −t ₂)·Q ₁₂;

and

Q ₁₂₃=(t ₅ −t)/(t ₅ −t ₃)·Q ₁₂+(t−t ₃)/(t ₅ −t ₃)·Q ₂₃.

Third internal division calculation:

Q ₀₁₂₃=(t ₄ −t)/(t ₄ −t ₃)·Q ₀₁₂+(t−t ₃)/(t ₄ −t ₃)·Q ₁₂₃.

Letting:

a ₀₀(t)=(t−t ₁)/(t ₄ −t ₁);

a ₁₀(t)=(t−t ₂)/(t ₅ −t ₂);

a ₂₀(t)=(t−t ₃)/(t ₆ −t ₃);

a ₀₁(t)=(t−t ₂)/(t ₄ −t ₂);

a ₁₁(t)=(t−t ₃)/(t ₅ −t ₃); and

a ₀₂(t)=(t−t ₃)/(t ₄ −t ₃),

then,

First internal division calculation:

Q ₀₁=(1−a ₀₀(t))Q ₀ +a ₀₀(t)Q ₁;

Q ₁₂=(1−a ₁₀(t))Q ₁ +a ₁₀(t)Q ₂; and

Q ₂₃=(1−a ₂₀(t))Q ₂ +a ₂₀(t)Q ₃.

Second internal division calculation:

Q ₀₁₂=(1−a ₀₁(t))Q ₀₁ +a ₀₁(t)Q ₁₂; and

Q ₁₂₃=(1−a ₁₁(t))Q ₁₂ +a ₁₁(t)Q ₂₃.

Third internal division calculation:

Q ₀₁₂₃=(1−a02(t))Q ₀₁₂ +a ₀₂(t)Q ₁₂₃.

For each of the types of the knot patterns, a knot vector T=[t₀, t₁, . .. , t₇] is defined as described below. Note that t₀ and t₇ may have anyvalue since they are not involved in the calculations. Internal divisionfactors determined for each of the knot vectors T will also be shownbelow.

Type 1:

Knot vector T=[−3, −2, −1, 0, 1, 2, 3, 4].

Here, the internal division factors are a₀₀(t)=(t+2)/3, a₁₀(t)=(t+1)/3,a₂₀(t)=t/3, a₀₁(t)=(t+1)/2, a₁₁(t)=t/2, and a₀₂(t)=t.

Type 2:

Knot vector T=[0, 0, 0, 0, 1, 1, 1, 1].

Here, the internal division factors are a₀₀(t)=t, a₁₀(t)=t, a₂₀(t)=t,a₀₁(t)=t, a₁₁(t)=t, and a₀₂(t)=t.

Type 3:

Knot vector T=[0, 0, 0, 0, 1, 2, 2, 2].

Here, the internal division factors are a₀₀(t)=t, a₁₀(t)=t/2,a₂₀(t)=t/2, a₀₁(t)=t, a₁₁(t)=t/2, and a₀₂(t)=t.

Type 4:

Knot vector T=[0, 0, 0, 0, 1, 2, 3, 4].

Here, the internal division factors are a₀₀(t)=t, a₁₀(t)=t/2,a₂₀(t)=t/3, a₀₁(t)=t, a₁₁(t)=t/2, and a₀₂(t)=t.

Type 5:

Knot vector T=[−1, −1, −1, 0, 1, 2, 2, 2].

Here, the internal division factors are a₀₀(t)=(t+1)/2, a₁₀(t)=(t+1)/3,a₂₀(t)=t/2, a₀₁(t)=(t+1)/2, a₁₁(t)=t/2, and a₀₂(t)=t.

Type 6:

Knot vector T=[−3, −2, −1, 0, 1, 1, 1, 1].

Here, the internal division factors are a₀₀(t)=(t+2)/3, a₁₀(t)=(t+1)/2,a₂₀(t)=t, a₀₁(t)=(t+1)/2, a₁₁(t)=t, and a₀₂(t)=t.

Type 7:

Knot vector T=[−1, −1, −1, 0, 1, 1, 1, 1].

Here, the internal division factors are a₀₀(t)=(t+1)/2, a₁₀(t)=(t+1)/2,a₂₀(t)=t, a₀₁(t)=(t+1)/2, a₁₁(t)=t, and a₀₂(t)=t.

Type 8:

Knot vector T=[−1, −1, −1, 0, 1, 2, 3, 4].

Here, the internal division factors are a₀₀(t)=(t+1)/2, a₁₀(t)=(t+1)/3,a₂₀(t)=t/3, a₀₁(t)=(t+1)/2, a₁₁(t)=t/2, and a₀₂(t)=t.

Type 9:

Knot vector T=[−3, −2, −1, 0, 1, 2, 2, 2].

Here, the internal division factors are a₀₀(t)=(t+2)/3, a₁₀(t)=(t+1)/3,a₂₀(t)=t/2, a₀₁(t)=(t+1)/2, a₁₁(t)=t/2, a₀₂(t)=t.

FIG. 19 summarizes the internal division factors for the respectivetypes of knot patterns. As can be seen, all the internal divisionfactors are given by linear expressions of t. During the tessellationprocessing, the parameter t takes on discrete values in the interval [0,1] at predetermined pitches depending on the number of divisions atwhich the parametric curve is divided into patches. To determine a pointon a B-spline curve requires that the values of the internal divisionfactors a_(ij)(t) be calculated for all the discrete values of theparameter t. The values of the internal division factors a_(ij)(t) forall the discrete values of the parameter t, however, need not beprepared in advance because the internal division factors a_(ij)(t) aregiven by the linear expressions of the parameter t. That is, if twovalues at t=0 and 1, or the initial value a_(ij)(0) and the final value(1), of each internal division factor are stored previously, a_(ij)(t)for any value of t (0<t<1) can be determined by interpolating theinitial value a_(ij)(0) and the final value a_(ij)(1) linearly at aratio of t:(1−t). In the present embodiment, this characteristic iseffectively utilized in the texture unit 70.

As has been described, according to the present embodiment, part of theinternal division calculations necessary for the recursive internaldivision operations of control points on a parametric surface isperformed by utilizing the linear interpolator of the texture unit 70.This improves the processing efficiency of the entire drawing processingapparatus 100 as compared to the cases where all the internal divisioncalculations are performed by using the arithmetic units of the pixelsshaders 32.

Among the values of the internal division factors which vary dependingon the two-dimensional parametric coordinate values, the initial valuesand final values alone are stored in the form of a texture. It istherefore possible to acquire the internal division factors for anytwo-dimensional parametric coordinate values efficiently byinterpolating the texture. Since the texture has only to contain theinitial values and final values of the internal division factors, it ispossible to suppress the memory capacity necessary for the tessellationprocessing. Moreover, the control points of the parametric surface canalso be stored as a texture, so that the first internal divisionoperation can be performed efficiently by interpolating the texture.

Up to this point, the present invention has been described inconjunction with the embodiment thereof. The foregoing embodiment hasbeen given solely by way of illustration. It will be understood by thoseskilled in the art that various modifications may be made tocombinations of the foregoing components and processes, and all suchmodifications are also intended to fall within the scope of the presentinvention. Some such modifications will be described below.

The foregoing description has dealt with the tessellation processing inwhich a parametric surface defined by a two-dimensional parametriccoordinate system is subdivided into meshes patch by patch.Nevertheless, tessellation processing in which a parametric curvedefined by a one-dimensional parametric coordinate system is subdividedinto lines segment by segment is also applicable. In the tessellationprocessing on a parametric curve, line primitives can be supplied to therasterizer 10 as drawing primitives.

The foregoing description has dealt with the case where the initialvalues a_(ij)(0) and the final values a_(ij)(1) of the internal divisionfactors dependent on the parameter t are linearly interpolated todetermine the values of the internal division factors a_(ij)(t)corresponding to a specified parameter t. Nevertheless, since theinternal division factors a_(ij)(t) are linear expressions of theparameter t, the internal division factor texture 54 is not limited tothe initial and final values but may contain internal division factorsa_(ij)(t₁) and a_(ij)(t₂) at any two different parameter values t₁ andt₂ as the representative values. Then, linear interpolation betweenthese representative values can be performed to determine the values ofinternal division factors a_(ij)(t) at the specified parameter t.Incidentally, this linear interpolation includes not only soleinterpolation but extrapolation as well.

The embodiment has dealt with the case where only the first internaldivision operation on the control points in the u direction is performedby using the linear interpolation function of the texture unit 70.Nevertheless, the second and third internal division operations may alsobe performed by using the linear interpolation function of the textureunit 70. In that case, another interface is provided so that the pixelshaders 32, after receiving the control points internally divided at thefirst internal division factors in advance from the texture unit 70, cansubject the internally-divided control points to the first texture unit70. The texture unit 70 further divides the control points subjectedfrom the pixel shaders 32 internally with the second internal divisionfactors by using the linear interpolator, and outputs the results to thepixel shaders 32. The pixel shaders 32 receive the control pointsinternally divided at the second internal division factors from thetexture unit 70, and subject the same to the texture unit 70 further.The texture unit 70 divides them internally with the third internaldivision factor further, and returns the results to the pixel shaders32. Consequently, all the internal division calculations necessary forthe tessellation processing can be achieved by using the linearinterpolation function of the texture unit 70. The linear interpolationpart 72 of the texture unit 70 may be configured so that the result oflinear interpolation can be fed back and input again.

1. A drawing processing apparatus comprising: a texture memory partwhich stores data on a texture; a texture processing part whichinterpolates the data on the texture stored in the texture memory partby using a linear interpolator based on a specified value of aparametric coordinate defined for each of units of drawing processing,thereby calculating interpolated data on the texture corresponding tothe specified value of the parametric coordinate; and a drawingprocessing part which processes the units of drawing processing by usingthe interpolated data on the texture, and wherein: the units of drawingprocessing are segments or component units of a parametric curverepresented by a one-dimensional parametric coordinate, or patches orcomponent units of a parametric surface represented by two-dimensionalparametric coordinates; the texture memory part stores a group ofrepresentative values of an internal division factor at two points in adomain of a parameter as the data on the texture, the internal divisionfactor depending on the parametric coordinate(s) defined for each of thesegments or patches; the texture processing part interpolates the groupof representative values of the internal division factor by using alinear interpolator with values of the parametric coordinate(s)successively specified by the drawing processing part as to a segment orpatch to be processed as interpolation factors, thereby calculatinginterpolated values of the internal division factor corresponding to thespecified values of the parametric coordinate(s); and the drawingprocessing part performs an internal division operation on a pluralityof control points defining shape of the segment or patch to be processedat ratios based on the interpolated values of the internal divisionfactor, thereby calculating a vertex coordinate or vertex coordinates onthe parametric curve or the parametric surface corresponding to thespecified values of the parametric coordinate(s) in the segment or patchto be processed.
 2. The drawing processing apparatus according to claim1, wherein the internal division operation of the drawing processingpart is one for internally dividing the plurality of control points inthe segment or patch recursively at ratios based on the interpolatedvalues of the internal division factor.
 3. The drawing processingapparatus according to claim 1, wherein: the texture memory part storesgroups of a plurality of control points defining the shape of therespective segments or patches; the texture processing part internallydivides the plurality of control points to be applied to the segment orpatch to be processed by using the linear interpolator with theinterpolated values of the internal division factor as interpolationfactors; and the subdivision processing part performs an internaldivision operation further on the plurality of control points internallydivided by the texture processing part, at ratios based on theinterpolated values of the internal division factor.
 4. A textureprocessing apparatus comprising: a texture memory part which stores dataon a texture; and a texture processing part which interpolates the dataon the texture stored in the texture memory part by using a linearinterpolator based on a specified value of a parametric coordinatedefined for each of units of drawing processing, thereby calculatinginterpolated data on the texture corresponding to the specified value ofthe parametric coordinate, and wherein; the units of drawing processingare segments or component units of a parametric curve represented by aone-dimensional parametric coordinate, or patches or component units ofa parametric surface represented by two-dimensional parametriccoordinates; the texture memory part stores groups of a plurality ofcontrol points defining shape of the respective segments or patches; andthe texture processing part acquires the group of a plurality of controlpoints to be applied to a segment or patch to be processed from thetexture memory part, and performs an internal division operation thereonby using the linear interpolator, thereby calculating a vertexcoordinate or vertex coordinates on the parametric curve or theparametric surface corresponding to the specified value(s) of theparametric coordinate(s) in the segment or patch to be processed.
 5. Thetexture processing apparatus according to claim 4, wherein the textureprocessing part subjects the plurality of control points internallydivided by the linear interpolator to the linear interpolator again,thereby performing a recursive internal division operation on theplurality of control points in the segment or patch.
 6. The textureprocessing apparatus according to claim 4, wherein: the texture memorypart stores a group of representative values of an internal divisionfactor at two points in a domain of a parameter in the parametriccoordinate(s) defined for the segment or patch as the data on thetexture, the internal division factor depending on the parametriccoordinate(s); and the texture processing part interpolates the group ofrepresentative values of the internal division factor by using thelinear interpolator with a value or values of the parametriccoordinate(s) specified in the segment or patch to be processed as aninterpolation factor or factors, thereby calculating an interpolatedvalue or values of the internal division factor corresponding to thespecified value(s) of the parametric coordinate(s), and performs aninternal division operation further on the plurality of control pointsby using the linear interpolator with the interpolated value(s) of theinternal division factor as an interpolating factor or factors, therebycalculating a vertex coordinate or vertex coordinates on the parametriccurve or parametric surface corresponding to the specified value(s) ofthe parametric coordinate(s) in the segment or patch to be processed. 7.A data structure of a texture having three-dimensional coordinate valuesof control points defining shape of a parametric surface as texelvalues, and the texture being configured so that, in response to atexture read instruction accompanied with a specified value of aparameter in a coordinate system of the parametric surface, thecoordinate values of adjoining two of the control points can be sampledto calculate interpolated values of the coordinate values of the controlpoints corresponding to the specified value.
 8. A data structure of atexture having initial values and final values of internal divisionfactors for use in a recursive internal division operation on controlpoints defining shape of a parametric surface are stored as adjoiningtexel values, and the texture being configured so that, in response to atexture read instruction accompanied with a specified value of aparameter in a coordinate system of the parametric surface, the initialvalues and the final values of the internal division factors can besampled to calculate interpolated values of the internal divisionfactors corresponding to the specified value.